The earlier applications mentioned above describe various beam-splitting polarizing systems three of which involve the use of prisms. These three arrangements may be summarized as follows:
1. The beam-splitting or light deviating means comprises a single, narrow-angle prism so located that it occupies one-half of the field of the projection lens, its apex edge being parallel to and adjacent the vertical diameter of the lens. The light polarizing means comprises two pieces of sheet polarizing material each of which occupies one-half of the field of the projection lens, the polarizing axes of the two pieces being mutually perpendicular and the two pieces being butted together so that the line of demarkation between them is parallel to and adjacent the vertical diameter of the projection lens.
2. The beam-splitting or light deviating means and the polarizing means are combined in the form of a single, narrow-angle, birefringent prism located adjacent the projection lens and occupying the complete field of the lens so that one of the two projected images is formed by ordinary rays and the other image is formed by extraordinary rays, the polarizing axes of the two sets of rays being mutually perpendicular.
3. The beam-splitting or light deviating means and the polarizing means are combined in the form of two prism components of birefringent material cemented together to constitute a plane-parallel plate in which the optic axes of the two component prisms are mutually perpendicular and parallel to the parallel faces of the plate. The device is located adjacent the projection lens and occupies the complete field of the lens so that one of the two projected images is formed by ordinary rays and the other image is formed by extraordinary rays, the polarizing axes of the two sets of rays being mutually perpendicular.
In each of these three arrangements the polarizing means is so mounted that it can be rotated through 180.degree. about the optical axis of the projection lens thus causing the polarizing axes of the two projected images to be interchanged. By this means the apparent position of the image seen through polarizing spectacles can be shifted from behind the screen to in front of the screen or vice versa.
The present invention is concerned with specific embodiments of the principles summarized above and has for its object the solution of two problems which arise in practice. The first of these problems lies in the fact that with many of the projectors being manufactured today, particularly those for the amateur or home movie market, the lens is recessed into the body of the projector in such a way that there is little clearance between the front of the lens and the projector housing, thus making it difficult, if not impossible, to attach any auxiliary optical device to the lens. The second problem arises from the fact that most projectors currently being manufactured for the home movie market are provided with zoom lenses, and such lenses cannot be used satisfactorily with optical systems of the types with which this invention is concerned. This is because, when the optical device is placed in front of a zoom lens, it is too distant from the exit pupil of the rear element. The effect is the same as that resulting from placing the device too far in front of a regular lens of fixed focal length. This so-called proximity effect is discussed in some detail in my earlier application, referenced above. Both of these problems are solved in accordance with the present invention by providing a compact optical assembly consisting of a projection lens of fixed focal length together with light deviating and polarizing means located inside the lens barrel.
For a proper understanding of the invention it is important to note a major difference between the light-deviating means employed in this case and the light-deviating means used in earlier systems for the projection of films each frame of which contains a pair of stereoscopically related images, each of these images occupying one-half of the film frame.
In the case of the present invention, the projected pictures comprise two similar, overlapping but laterally displaced, pictures of a single image which occupies the entire film frame. It is unnecessary for the lateral displacement of the projected images at the screen surface ever to be appreciably in excess of the human interocular separation, that is, say, in excess of about three inches. This means, in practice, that the deviation produced by the single prism employed needs to be only about 0.degree. 06' for long projector-to-screen distances, such as 100 feet, or 1.degree. 00' for short projector-to-screen distances, such as 10 feet. As explained in detail in Copending Application Ser. No. 518,355, filed on Oct. 29, 1974, the light-deviating and polarizing means must be located as close to the front of the lens as possible in order that the proximity effect may be a maximum.
Referring, now, to the earlier systems involving the projection of two half-frame images, the angular deviation produced by the prisms is necessarily of much greater magnitude. The deviation must be equal to one-half the field angle of the projection lens. Thus, for example, if the field angle is 12.degree., the angular deviation must be 6.degree., regardless of the projector-to-screen distance. So large a deviation cannot be produced by means of a single prism located in one-half of the lens field; this is because the deviated image would suffer from severe chromatic aberration and because its horizontal dimension would be unduly compressed, thereby rendering binocular fusion of the two projected images impossible for an observer to accomplish. Consequently, it is necessary to employ two prisms, one in each half of the lens field, each prism producing, in opposing directions, an angular deviation equal to one-quarter of the field angle of the projection lens. The two prisms, together with the polarizers, should be located as far from the front of the lens as possible in order that the proximity effect may be a minimum. Typically, in the case of a lens having a field angle of approximately 12.degree., for use with 16 mm film, the distance of the prisms and the polarizers from the front of the lens should be about one foot. If the prisms and polarizers be located too close to the front of the lens, then, in addition to the wanted, overlapping stereoscopic pair of images, two unwanted, spurious images will be projected, one on each side of the wanted stereoscopic pair. In order to eliminate these spurious images, it is necessary to insert an additional pair of polarizers in the film gate of the projector; this is a physical impossibility without a projector specially designed for the purpose. Thus, it will be obvious that a system for projecting half-frame stereoscopic pairs cannot be manufactured in the form of a compact optical assembly as described herein.